Using the return value of strlcpy() and strlcat()

Hi hackers,

Over in the "Include all columns in default names for foreign key
constraints" thread[1], I noticed the patch added the following:

+ strlcpy(buf + buflen, name, NAMEDATALEN);
+ buflen += strlen(buf + buflen);

Seeing as strlcpy() returns the copied length, this seems rather
redundant. A quick bit of grepping shows that this pattern occurs in
several places, including the ChooseIndexNameAddition and
ChooseExtendedStatisticNameAddition functions this was no doubt inspired
by.

Attached is a patch that instead uses the return value of strlcpy() and
strlcat(). I left some strlen() calls alone in places where it wasn't
convenient (e.g. pg_open_tzfile(), where it would need an extra
variable).

- ilmari

[1] https://postgr.es/m/CAF+2_SHzBU0tWKvJMZAXfcmrnCwJUeCrAohga0awDf9uDBptnw@mail.gmail.com
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