Add append functions for char * and a single char.

* utstring_append_len(UT_string *dst, const char *src, size_t len):
 appends up to `len` bytes from `src` to `dst` expanding `dst` if
 needed, '\0' is added to the resulting string automatically.

* utstring_append_c(UT_string *dst, char c):
 appends a character `c` to `dst` expanding `dst` if needed, '\0' is
 added to the resulting string automatically.
Do not redefine oom function definition.