core: Implement and export to plugins string_strconcat()
core: Prefer string_strconcat to sprintf for filename completions

This also avoids the use of PATH_MAX
fifo: Prefer weechat_string_strconcat to snprintf

This also avoids the use of PATH_MAX
core: Implement and export to plugins util_realpath()
core: Prefer util_realpath() to realpath() when writing config file

This also avoids the use of PATH_MAX
script: Prefer util_realpath() to realpath() when loading scripts

This also avoids the use of PATH_MAX
core: Add global weechat_dir_absolute_path

This also avoids the use of PATH_MAX
core: Remove PATH_MAX defines, as is no longer needed.