Improve description of loread/lowrite.

Patch by me, reviewed by Tatsuo Ishii.
Fix unescaping of JSON Unicode escapes, especially for non-UTF8.

Per discussion  on -hackers. We treat Unicode escapes when unescaping
them similarly to the way we treat them in PostgreSQL string literals.
Escapes in the ASCII range are always accepted, no matter what the
database encoding. Escapes for higher code points are only processed in
UTF8 databases, and attempts to process them in other databases will
result in an error. \u0000 is never unescaped, since it would result in
an impermissible null byte.
Improve updatability checking for views and foreign tables.

Extend the FDW API (which we already changed for 9.3) so that an FDW can
report whether specific foreign tables are insertable/updatable/deletable.
The default assumption continues to be that they're updatable if the
relevant executor callback function is supplied by the FDW, but finer
granularity is now possible.  As a test case, add an "updatable" option to
contrib/postgres_fdw.

This patch also fixes the information_schema views, which previously did
not think that foreign tables were ever updatable, and fixes
view_is_auto_updatable() so that a view on a foreign table can be
auto-updatable.

initdb forced due to changes in information_schema views and the functions
they rely on.  This is a bit unfortunate to do post-beta1, but if we don't
change this now then we'll have another API break for FDWs when we do
change it.

Dean Rasheed, somewhat editorialized on by Tom Lane
Don't use ordinary NULL-terminated strings as Name datums.

Consumers are entitled to read the full 64 bytes pertaining to a Name;
using a shorter NULL-terminated string leads to reading beyond the end
its allocation; a SIGSEGV is possible.  Use the frequent idiom of
copying to a NameData on the stack.  New in 9.3, so no back-patch.
Observe array length in HaveVirtualXIDsDelayingChkpt().

Since commit f21bb9cfb5646e1793dcc9c0ea697bab99afa523, this function
ignores the caller-provided length and loops until it finds a
terminator, which GetVirtualXIDsDelayingChkpt() never adds.  Restore the
previous loop control logic.  In passing, revert the addition of an
unused variable by the same commit, presumably a debugging relic.
Don't pass oidvector by value.

Since the structure ends with a flexible array, doing so truncates any
vector having more than one element.  New in 9.3, so no back-patch.
Avoid reading below the start of a stack variable in tokenize_file().

We would wrongly overwrite the prior stack byte if it happened to
contain '\n' or '\r'.  New in 9.3, so no back-patch.
Avoid reading past datum end when parsing JSON.

Several loops in the JSON parser examined a byte in memory just before
checking whether its address was in-bounds, so they could read one byte
beyond the datum's allocation.  A SIGSEGV is possible.  New in 9.3, so
no back-patch.
Only install a portal's ResourceOwner if it actually has one.

In most scenarios a portal without a ResourceOwner is dead and not subject
to any further execution, but a portal for a cursor WITH HOLD remains in
existence with no ResourceOwner after the creating transaction is over.
In this situation, if we attempt to "execute" the portal directly to fetch
data from it, we were setting CurrentResourceOwner to NULL, leading to a
segfault if the datatype output code did anything that required a resource
owner (such as trying to fetch system catalog entries that weren't already
cached).  The case appears to be impossible to provoke with stock libpq,
but psqlODBC at least is able to cause it when working with held cursors.

Simplest fix is to just skip the assignment to CurrentResourceOwner, so
that any resources used by the data output operations will be managed by
the transaction-level resource owner instead.  For consistency I changed
all the places that install a portal's resowner as current, even though
some of them are probably not reachable with a held cursor's portal.

Per report from Joshua Berry (with thanks to Hiroshi Inoue for developing
a self-contained test case).  Back-patch to all supported versions.