POSIX-ify the GNUPGHOME variable

[chawyehsu]

When GnuPG is built for MSYS2 (e.g., as part of Git for Windows), it expects
the GNUPGHOME environment variable to contain a POSIX-style path
(e.g., /c/Users/user/.gnupg). However, users often set this variable using a
Windows-style path (e.g., C:\Users\user\.gnupg), which is the native format
for GnuPG for Windows.

This discrepancy leads to MSYS2-based GnuPG failing to correctly locate its home
directory when running within an environment having the GNUPGHOME defined in
the Windows-style, as the Windows path is not properly interpreted. This issue
prevents users from seamlessly sharing a single GnuPG home directory across
different GnuPG installations (native Windows build vs. MSYS2-based). 1 2

To resolve this, we posix-ify GNUPGHOME in a manner similar to SHELL. 3
This change will allow MSYS2-based GnuPG to correctly interpret the GNUPGHOME
variable regardless of whether it's set with a Windows path, improving the
interoperability and user experience.

Issue 4 and 5 from git-for-windows should benefit from this change.

This addresses #297

[jeremyd2019]

Has anyone considered that win32 gpg and cygwin gpg won't be able to talk to each other? cygwin will be using their unix sockets, and has its own pid namespace, so it seems likely that you could get two different gpg-agents and/or dirmngrs running on the same GNUPGHOME, and I don't know what sort of issues that could cause.

[dscho]

win32 gpg and cygwin gpg won't be able to talk to each other

That's a really good point I had not considered before.

My thinking involved the keyring only, which should be able to be shared. @chawyehsu could you test what happens when you start a Win32 GPG agent and try to use it via the MSYS2 GPG, and also what happens when you start an MSYS2 GPG agent and try to use it via Win32 GPG? (You should be able to perform these tests by installing mingw-w64-x86_64-gnupg and gnupg side by side.)

[jeremyd2019]

I seem to remember that mingw-w64-gnupg didn't actually work right... probably better to use gpg4win.org (or the win32 package of gnupg)

I was thinking, maybe gnupg needs the same sort of treatment as ssh (do you want to use the msys2 version or the win32 version?), though I hate the idea of yet another GfW installer option