scripts: Remove -a/-all switch
scripts: Fix typo
scripts: Fix site-specific installation
mod_admin_modules: Export install function, and minor fixes
scripts: Check for existence of scm command
mod_admin_modules: Handle error messages gracefully
core: Add exec_zmm for calling bin/zmm

This works by opening an Erlang port to bin/zmm and
wrapping the call in a spawn so that modules are installed asynchronously.
mod_admin_modules: Add pagination to ZMM listings