core: Not the configured but granted number of possible file descriptors.

With CLI "core show settings", simply the parameter maxfiles of the file
asterisk.conf was shown. If that parameter was not set, nothing was displayed
although the environment might have set a default number itself. Or if maxfiles
were not granted (completely), still maxfiles was shown. Now, the maximum number
of possible file descriptors in the environment is shown.

ASTERISK-26097

Change-Id: I2df5c58863b5007b34b77adbe28b885dfcdf7e0b