mingw: Add platform checking of GoDocBrowser for Cygwin on windows

[rwxguo]

• When using Cygwin/MSYS2 on windows, the :GoDocBrowser cannot popup the browser correctly.
  • Added has('win32unix') for platform compatibility.
• Also resolved vim screen mess up after using :silent with external command
  • refer to :h :silent
  • https://vi.stackexchange.com/questions/2809/silent-makes-my-vim-go-blank

[bhcleek]

Thank you

[rwxguo]

welcome, happy holiday !

[rwxguo]

@bhcleek Thank you for your guide and instruction. I added cygwin detection go#util#IsCygwin.

The reason using uname to check the platform is inspired by the maven-wrapper project, which is widely used in enterprise level java projects using SpringBoot etc.

Plus, I also leverage the vim built-in feature list has('win32unix') to make it more concise.

I have tested these factors on three major cygwin platforms, the result is for your reference:

platform	has('win32')	has('win64')	has('win32unix')	uname
Cygwin	0	0	1	CYGWIN_...
GitBash (64bit)	0	0	1	MINGW64_...
MSYS2 - mingw32.exe	0	0	1	MINGW32_...
MSYS2 - mingw64.exe	0	0	1	MINGW64_...
MSYS2 - msys2.exe	0	0	1	MSYS_...

[rwxguo]

By the way, I found there is a util function go#util#IsUsingCygwinShell which might not correct, because it uses go#util#IsWin, but based on my above testing result, go#util#IsWin always return 0 on all cygwin-like platform...

 " Checks if using:
 " 1) Windows system,
 " 2) And has cygpath executable,
 " 3) And uses *sh* as 'shell'
function! go#util#IsUsingCygwinShell()
  return go#util#IsWin() && executable('cygpath') && &shell =~ '.*sh.*'
endfunction

The go#util#IsUsingCygwinShell is used in path.vim, for your information

[bhcleek]

Yes, go#util#IsUsingCygwinShell is for a different kind of use case than you've been working on. The last I checked, it is correct for the use case for which it is intended.

[rwxguo]

As CYGWIN uses cygstart instead of start, but MSYS2 and GitBash uses start, I made the cygwin checking as a seprated "elseif", and use rundll32 instead of start rundll32 or cygwin rundll32. (treat cygwin as a separated system, rather than a windows variation)