runtime: don't execute external commands when loading ftplugins #373

Job	Run time
linux (tiny, clang, native)	1m 55s
linux (tiny, gcc, native)	2m 9s
linux (normal, clang, native)	6m 17s
linux (normal, gcc, native)	6m 22s
linux (huge, clang, native)	8m 28s
linux (huge, gcc, native)	15m 36s
linux (tiny, clang, nogui)	2m 6s
linux (tiny, gcc, nogui)	2m 7s
linux (normal, gcc, i386, ./src/shadow)	10m 50s
linux (huge, gcc, uchar, testgui, true, dynamic)	13m 9s
linux (huge, clang, asan, 5.1)	25m 42s
linux (huge, gcc, unittests, true)	2m 58s
linux (normal, gcc, vimtags)	2m 21s
macos (tiny)	1m 39s
macos (normal)	10m 46s
macos (huge)	17m 8s
windows (HUGE, msvc, no, no, x64, stable)	7m 42s
windows (HUGE, mingw, yes, yes, x86, stable, yes)	14m 50s
windows (HUGE, msvc, no, yes, x86)	12m 21s
windows (HUGE, mingw, yes, no, x64, yes)	33m 30s
windows (NORMAL, msvc, yes, no, x86)	8m 31s
windows (NORMAL, mingw, no, yes, x64)	10m 35s
windows (TINY, msvc, yes, yes, x64)	2m 49s
windows (TINY, mingw, no, no, x86)	3m 29s
	3h 43m 20s

Provide feedback