archiso/mkarchiso:
Change `_pacman()` to use the *modified* pacman.conf from the work_dir, instead of using the *unmodified* pacman.conf from
the profile.

Change `_make_pacman_conf()` to compare the system's and the profile's CacheDir setting and use the profile's CacheDir
setting only if it's not the default and not the same as the system's.

Always set the HookDir to the airootfs' override directory, so that no hooks from the host system are being run.

Remove DBPath, LogFile and RootDir settings from the work_dir pacman.conf as they are otherwise referring to the host
system, **even if** pacman is being called with the `-r` flag.

Fix a typo in _make_custom_airootfs().

README.profile.rst:
Add information about the pacman.conf in a profile and how configuration options behave, when used by mkarchiso.

Fixes #73
Fixes #74