Use pkgbuild-mode, change sroll one line keybindings

Use the pkgbuild major mode when editing PKGBUILDs.  Change the
scroll up/down one line keybindings to M-n/M-p to mimic the
keybindings for next/previous line (C-n/C-p).

Signed-off-by: Chirantan Ekbote <chirantan.ekbote@gmail.com>