Drop the "--no-exec-mask" flag and function parameters.

Nobody needs the "--no-exec-mask" flag, and it's uglying up the
library's API. Sayonara:

  * Update the man page:
    * Remove all mentions of the flag.
    * Update the algorithm description.
    * Reword the general description.
  * Remove all --no-exec-mask tests.
  * Bump the program version in configure.ac.
  * Make apply_default_acl() work as if no_exec_mask == false.
  * Remove all no_exec_mask function parameters.
  * Bump the soname major version in src/Makefile.am.