MINOR: tools: make csv_enc_append() always start at the first byte of the chunk

MINOR: tools: make csv_enc_append() always start at the first byte of the chunk

csv_enc_append() returns a pointer to the beginning of the encoded
string, which makes it convenient to use in printf(). However it's not
convenient for use in chunks as it may leave an unused byte at the
beginning depending on the automatic quoting. Let's modify it to work
in two passes. First it looks for a character that requires escaping
using strpbrk(), and second it encodes the string. This way it
guarantees to always start at the first available byte of the chunk.
Additionally it made the code quite simpler.