astfd: With RLIMIT_NOFILE only the current value is sensible.

With menuselect "DEBUG_FD_LEAKS" and CLI "core show fd", both the maximum max
and current max of possible file descriptors were shown. Both show the same
value always. Not to confuse users, just the current maximum is shown now.

ASTERISK-26097

Change-Id: I49cf7952d73aec9e3f6a88942842c39be18380fa