Split only at first equal sign more often

[stoeckmann]

Most of the split calls for the '=' character have a maxsplit set to 1 with only a few exceptions. These should be no exceptions to avoid unintended behavior.

Example output with adjusted test case:

MY_COMPILER_ENV=value ./test\ cases/common/49\ custom\ target/my_compiler.py --input=input=file --output=output=file /bin/ls

Traceback (most recent call last):
  File "/home/user/projects/meson/./test cases/common/49 custom target/my_compiler.py", line 16, in <module>
    with open(args[1].split('=')[1]) as f:
         ~~~~^^^^^^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: 'input'

According to my given arguments, input=file should have been opened, not input.