This is the last direct getenv() call in the tree (besides the one in
os_getenv()); most of the work was already done in [1].

[1]: 412d246be71bd99cb4edde4e6f984b0b0d91bcd9

Some more fixes to XDG code

The assertion now considers the case where events are disabled.

This makes it impossible for K_EVENT to interfere with mappings, but it also
disables processing of events while in the middle of a mapping (Though this will
be fixed later as this refactoring progresses).

`may_sync_undo` is now called when K_EVENT is received. This is necessary to
correctly update undo entry lists before executing some action.

Refactor input.c, normal.c and edit.c to use the K_EVENT special key to trigger
the CURSORHOLD event. In normal and edit mode, K_EVENT is treated as
K_CURSORHOLD, which enables better handling of arbitrary actions in those
states(eg: In normal mode the previous operator counts will be restored).

Also fix a test in vim_spec.lua. The test had a wrong assumption: cmdheight is
only used to determine when the press enter screen will be shown, not to limit
how many lines or control pagination.

This way all standard Vim file paths have .vim extension. VIMRC_FILE constant 
used for &exrc option was not touched.