pgsql: Rethink API for pg_get_line.c, one more time.

Rethink API for pg_get_line.c, one more time.

Further experience says that the appending behavior offered by
pg_get_line_append is useful to only a very small minority of callers.
For most, the requirement to reset the buffer after each line is just
an error-prone nuisance. Hence, invent another alternative call
pg_get_line_buf, which takes care of that detail.

Noted while reviewing a patch from Daniel Gustafsson.

Discussion: https://postgr.es/m/48A4FA71-524E-41B9-953A-FD04EF36E2E7@yesql.se

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/931487018c409a3102452f965ccaa48367244a41

Modified Files
--------------
src/bin/initdb/initdb.c | 4 +---
src/common/pg_get_line.c | 29 ++++++++++++++++++++++++++++-
src/include/common/string.h | 1 +
src/interfaces/ecpg/test/pg_regress_ecpg.c | 3 +--
src/test/regress/pg_regress.c | 3 +--
5 files changed, 32 insertions(+), 8 deletions(-)