Avoid unneeded warning when scanning JOB_TMPDIR_MNTDIR for possible
leftover job mounts. The parent directory of JOB_TMPDIR_MNTDIR is
created when the first job with the --tmpdir feature is started. So
it won't exist in a new server.

This cleanup path is only used in the exceptional case that the usual
cleanup path (via job_has_finished) didn't succeed, e.g. when mxqd was
killed. The cleanup is not essential for the currently running server.

Add function attribute ((unusged)) to avoid a compiler warning when the
static inline function defined in the header file is not used by the
compilation unit.

Add header file for the quasi-standard [1] xmalloc call.

Include x-version of strndup. Other functions (e.g. xrealloc) can be
added as needed.

The current approach implemented in mx_util is to wait and retry on
ENOMEM malloc failure. However, this overhead doesn't seem to be
justified, because it is difficult to imagine a case, where a malloc
would fail with ENOMEM at one time and a retry would succeed.

[1] https://www.gnu.org/software/libc/manual/html_node/Malloc-Examples.html

Add a utility module which can store a set of keywords. The set can be
created and update from a string and can be serialized into a string.

The string representation is a space-separated list of keywordis. Strings
created by this utility contain the keywords stored in the set sorted in
lexical order separated by a single space character.

Input strings used to create or update a keyword set contain keywords
separated by whitespace. Words from the string are added to the set
unless a word starts with '-' in which case it will be removed from the

Usage example:

    struct keywordset *kws = keywordset_new("xxx yyy")
    keywordset_update(kws, "-yyy aaaa")

    char *s = keywordset_get(kws):   // s now "aaaaa xxx"
    free(s);                         // caller must free()

    if (keywordset_ismember(kws,"xxx")) ... // true

    keywordset_free(kws);

Add a new option so that specific servers can be excluded from starting
jobs for this group. E.g.:

    mxqsub --disabled-servers="dontpanic sigusr2" sleep 100

Add a new command which can be used to modify an existing group:

    mxqset group 123 --closed
    mxqset group 123 --open
    mxqset group 123 --disabled-servers=dontpanic
    mxqset group 123 --update-disabled-servers="sigusr sigusr2 -dontpanic"
    mxqset group 123 --disabled-servers=

The flags open and closed can be set from mxqadmin as well
(`mxqadmin --close=123`) , but the synopsis, in which the options take
the groupid as a value is difficult to expand to options, which take a
value by themself.

Check, whether this server is qualified to start jobs from a group.
Lazy-evaluate qualification criteria and cache the result.
Don't start jobs we are not qualified for.

For now, the only qualification criteria is, that our short or fully
qualified hostname is not included the the excluded_servers set of the
group.

This can later be expanded to additional criteria (e.g. hostconfig or
processor flags).

The optimizing compiler predicts that pointer != NULL is more likely
than pointer == NULL anyway. The generated code by "gcc -O2" is the
same with or without __builtin_expect builtin.

However, probably based on special knowledge of malloc either because of
its __attribute__ ((malloc)) declaration or hard-coded, the optimization
without the __builtin_expect builtin seems to be a bit more stronger:
The function out_of_memory() is put into a .text.unlikely code segment,
as if had attribute ((cold)) had been declared for it.

Remove __builtin_expect.