weechat: Remove all unnecessary use of PATH_MAX

wee-util: Add util_realpath() which only uses PATH_MAX if necessary
wee-string: Add string_strconcat() which concats variadic strings
wee-completion: Use string_strconcat instead of sprintf()
wee-config-file: Use util_realpath()
weechat.c: Add global weechat_dir_absolute_path
plugin-api: Use global weechat_dir_absolute_path
plugins: Export util_realpath() and string_strconcat()
fifo: Use string_strconcat()
script: Use util_realpath()