tool_findfile: search for a file in the homedir

tool_findfile: search for a file in the homedir

The homedir() function is now renamed into findfile() and iterates over
all the environment variables trying to access the file in question
until it finds it. Last resort is then getpwuid() if
available. Previously it would first try to find a home directory and if
that was set, insist on checking only that directory for the file. This
now returns the full file name it finds.

The Windows specific checks are now done differently too and in this
order:

1 - %USERPROFILE%
2 - %APPDATA%
3 - %USERPROFILE%\\Application Data

The windows order is modified to match how the Windows 10 ssh tool works
when it searches for .ssh/known_hosts.

Reported-by: jeffrson on github
Co-authored-by: Jay Satiro
Fixes #8033
Closes #8035