Implement initial support for XDG [1]

Introduce getFontForgeUserDir(arg) where arg is one of Cache, Config, or
Data.  Replace every instance of getPfaEditDir(buffer) with
getFontForgeUserDir(Config).

[1] http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
Merge branch 'xdg_base_dir_spec' of https://github.com/holomorph/fontforge into holomorph-xdg_base_dir_spec
remove getPfaEditDir()
Implement initial support for XDG 2

* change getFontForgeUserDir to return NULL on failure
* add comments to document new functions getUserHomeDir, smprintf, and
  getFontForgeUserDir
Merge pull request #993 from mskala/xdg_cleanup

remove getPfaEditDir()
Merge pull request #992 from holomorph/xdg_base_dir_spec

Document initial support for XDG
try to match existing style

also remove superfluous function getDotFontForgeDir
Merge pull request #994 from holomorph/xdg_base_dir_spec

try to match existing style
Remove some resource leaks in fontforge/fvimportbdf.c - free some memory.