Xpra assumes all Linuxes have which installed, resulting in errors when its not

[miojonka]

Xpra currently checks for its own binary when connecting to a server by using which xpra. But it does not verify that the program which is actually installed (probably because many distributions make part of their base install).

This causes several problems:

• not every distribution has which installed (it is not part of Archlinux base)
• when which is not installed, xpra claims that Xpra is not installed instead upon connecting
• which is not a proper way to find the path to a binary - command -v is. (POSIX)

#3215 should fix this.

[totaam]

Out of curiosity, which OS are you running on?
which is installed on every distribution that I have used for testing.
Please see https://github.com/Xpra-org/xpra/pull/3215/files#r674875670

[miojonka]

I'm running Archlinux (archlinux.org) which does not have which installed in it's base package.