Introduce elib.callcmd() with more flexibility

Introduce elib.callcmd() with more flexibility

Commands can now be passed as argument vectors straight into execvp().
This removes all shell quoting headaches when going through this
interface.
File descriptors can be set up in a table-driven way as necessary.
A workdir for the child can be set. Environment variables can be
supplied now.

All callcmd friends are either removed or converted to this interface
with the exception of callcmd_pipe().

Signed-off-by: Tobias Ulmer <tu@emlix.com>