moved F_NOCACHE to a new header file to help port fsx.c

[kirussel]

I propose the addition of a new header file, fsx.h, to help port fsx.c to other platforms. My changes compile on "Darwin Kernel Version 14.0.0" and "FreeBSD 10.1-STABLE". The biggest change is moving F_NOCACHE to the header file.

My goals were:

• to keep any conditional compilation noise confined to fsx.h, instead of putting this in fsx.c
• to keep the functionality of fsx.c the same as the original
• avoid using conditional compilation to prevent parts of fsx.c code from compiling on other platforms

References:

• https://fstools.macosforge.org/trac/ticket/15
• master...kirussel/fsxh