Allow querying font by name for its height and other info. (Bug#19395)

Allow querying font by name for its height and other info.  (Bug#19395)

 src/font.c (Ffont_info): Add more font information to the vector
 returned by the function, inspired by query-font.  Doc fix.

 doc/lispref/display.texi (Low-Level Font): Document font-info and query-font.

 lisp/international/mule-diag.el (describe-font-internal): Display
 additional info returned by font-info.
 lisp/linum.el (linum--face-width): Rename from linum--face-height,
 and use the new functionality of font-info.
 (linum-update-window): Use linum--face-width and frame-char-width,
 instead of approximating with height.

 etc/NEWS: Mention the enhancement in font-info.