[conf, main] add support for and preliminary implementation of verbose font definers

The definers ``info_patch`` and ``info_generic`` wrap the two existing
font definers in a function that emits verbose information about the
definitions taking place.
[auxiliary] robustify font property lookups

This adresses an issue with entries in the font table that lack the
common font structure (manually parsed fonts??). Subtable lookups
are now guarded with nil-checks.

Reported by Herbert Voss

Signed-off-by: Philipp Gesang <phg42.2a@gmail.com>
[auxiliary] eliminate redundant conditions
Merge branch 'master' of github:phi-gamma/luaotfload

Conflicts:
	src/luaotfload-main.lua