This gets rid of the duplicate ldlinux.c32 and the useless isolinux.cfg which only points to syslinux.cfg.

Implements archlinux/archiso#46 .

profiledef.sh can now contain an associative array called file_permissions which can be used to set custom ownership and mode of custom airootfs files. The array's keys contain the path and the value is a colon separated list of owner UID, owner GID and access mode.
For example:

    file_permissions=(
      ["/etc/shadow"]="0:0:400"
    )

This means that mkarchiso now copies airootfs files (and directores) without permissions and anything that should be owned by a user other than root and/or if the mode should be something other than 644 for files and 755 for directories must to be listed in ${file_permission[@]} in profiledef.sh.

Fixes archlinux/archiso#61 .

mkarchiso: add xorrisofs options from boot mode specific functions instead of hardcoding them in _build_iso

README.profile.rst:
Add information on all necessary components of a profile:

* profiledef.sh
* packages.arch
* airootfs/
* efiboot/
* isolinux/
* syslinux/

Fix wording for pacman.conf documentation

README.rst:
Add a link to README.profile.rst.

Fixes #38

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