input_nmsg: rename ulen -> u_len, ubuf -> u_buf
dlmod: _nmsg_dlmod_init(): don't relativize the path to the DSO
msgmodset: _nmsg_msgmodset_init(): don't change directories

previously, _nmsg_msgmodset_init() would save the current working
directory, change to the plugin directory, load plugins, then change to
the saved working directory. instead, generate the absolute path
to the plugin files and load them without changing directory.