Use POSIX-compliant function names in bash completion

Use POSIX-compliant function names in bash completion

When running in posix mode (for example, because it was invoked as `sh`,
or with the --posix option), bash rejects the function names previously
used because they contain hyphens, which are not legal POSIX names, and
exits immediately.

This is a particularly serious problem on a system in which the
following three conditions hold:

1. The `sh` executable is provided by bash, e. g. via a symlink
2. Gnome Display Manager is used to launch X sessions
3. Bash completion is loaded in the (system or user) profile file
   instead of in the bashrc file

In that case, GDM's Xsession script (run with `sh`, i. e., bash in posix
mode) sources the profile files, thus causing the shell to load the bash
completion files. Upon encountering the non-POSIX-compliant function
names, bash would then exit, immediately ending the X session.

Fixes #521.

Signed-off-by: Lucas Werkmeister <mail@lucaswerkmeister.de>