pgsql: Improve some ancient, crufty code in bootstrap + initdb.

Improve some ancient, crufty code in bootstrap + initdb.

At some point back in the last century, somebody felt that reading
all of pg_type twice was cheaper, or at least easier, than using
repalloc() to resize the Typ[] array dynamically. That seems like an
entirely wacko proposition, so rewrite the code to do it the other
way. (To add insult to injury, there were two not-quite-identical
copies of said code.)

initdb.c's readfile() function had the same disease of preferring
to do double the I/O to avoid resizing its output array. Here,
we can make things easier by using the just-invented pg_get_line()
function to handle reading individual lines without a predetermined
notion of how long they are.

On my machine, it's difficult to detect any net change in the
overall runtime of initdb from these changes; but they should
help on slower buildfarm machines (especially since a buildfarm
cycle involves a lot of initdb's these days).

My attention was drawn to these places by scan-build complaints,
but on inspection they needed a lot more work than just suppressing
dead stores :-(

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e0f05cd5ba76a75e2ce3b85ba050e48e857dca00

Modified Files
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src/backend/bootstrap/bootstrap.c | 119 +++++++++++++++++++-------------------
src/bin/initdb/initdb.c | 46 +++++----------
2 files changed, 74 insertions(+), 91 deletions(-)