run_command: improve performance for logging and long lines

run_command: improve performance for logging and long lines

to call out/err/logfunc with each line, we search for a newline and call
outfunc/logfunc with everything before that

since we do a select/read (with 4096 size) in a loop, this means
that if we have very long lines, we search for a newline in an
ever growing buffer (for which we know does not contain a newline)

so instead, only search the new data for newlines

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>