trace2: do not use strbuf_split*()

trace2: do not use strbuf_split*()

tr2_cfg_load_patterns() and tr2_load_env_vars() functions are
functions with very similar structure that each reads an environment
variable, splits its value at the ',' boundaries, and trims the
resulting string pieces into an array of strbufs.

But the code paths that later use these strbufs take no advantage of
the strbuf-ness of the result (they do not benefit from <ptr,len>
representation to avoid having to run strlen(<ptr>), for example).

Simplify the code by teaching these functions to split into a string
list instead; even the trimming comes for free ;-).

Signed-off-by: Junio C Hamano <gitster@pobox.com>