Use strbuf for reading configuration files

Use strbuf for reading configuration files

Use struct strbuf from Git instead of fixed-size buffers to remove the
limit on the length of configuration file lines and refactor
read_config_line() to improve readability.

Note that this also fixes a buffer overflow that existed with the
original fixed-size buffer implementation.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>