Get rid of multiple compat.exec_command* variants

This is a good first issue: No PyInstaller-spefic knowledge is required, but you need to have a good understanding of encodings and different implementations between Python 2 and Python3 (and maybe between different versions of subprocess).

In compat there are several variants of exec_command:

• exec_command_rc: used twice (plus once int the old test-suite)
• exec_command_stdout: used twice (in hooks)
• exec_command_all: used once
• exec_command: used about 15 times.

The aim is to remove some of these or make them slim layers around each other. Unfortunately these are not building on top of each other, but are wrapping different subprocess.* functions.

Todo:

• Carefully read the documentation (in the code) and notice differences.
• Read all commit messages related to these functions.
• Check if some may trivially be merged.
• Try to build test-cases
• Clean them up.

Prior to providing a pull-request, please drop a note and we will prepare an extra branch for this.

Start in the compat module by reading the in-code documentation for exec_command_rc, exec_command_stdout, exec_command_all, and exec_command, then inspect their uses in hooks and the old test suite. Read all related commit messages and build test cases covering their encoding, subprocess, and return-value differences; done means redundant variants are removed or reduced to thin layers and the tests are cleaned up.