docfind: Use STARLINK_DIR environment variable to allow relocation

This is preferred over hard-coding @prefix@ into the script at build
time.
(cherry picked from commit fc5ea97eb3af4e084ddecc0d870b619b13031339)
specx: Fix creation of dynamic symbols on 64-bit

The ability to create symbols dynamically was completely broken
as the relevant code in gen_make_var and scl_symbols was failing
to understand that some pointer arithmetic was happening.

UMEMORY_PTR is a block of memory that was having variables assigned
into it and a byte offset was being used to work out where to put
each new set of points. A pointer was then calculated and passed to
the symbol creation routines.

Obviously each pointer was unknown to CNF so now we have to retrieve
the actual pointer from UMEMORY_PTR, add on the offset, and then
register the new pointer with CNF. This code also switches us to 8-byte
boundaries instead of 4-byte so we will be using more memory on
32-bit.
(cherry picked from commit 1b122c474ac93b7f1d1da1e6def6b4d908220fef)
smurf: Recalculate WVM tau in output headers

The WVMTAUST and WVMTAUEN headers are calculated using the current
conversion values at the summit when the data are taken. These
values are usually incorrect as we have improved the WVM calculation
significantly over time. To make it more obvious which values are
being used for extinction correction internally we now recalculate
the WVM tau and update the relevant FITS headers.

This patch does not update the associated WVM times.
(cherry picked from commit d0c291b6d96958b9bc9a9ed66583562166fc612a)
smurf: Update WVM start and end time FITS headers in output map
(cherry picked from commit b5dea7f1931b76cb1fd80abfb6f6dbd0dd56237e)
smurf: Annul error caused by unusable airmass info when updating WVM headers

Eg flatfields have unusable airmass info, but we do not want this to
prevent the whole application from running.
(cherry picked from commit 64df832c34826c928445b64818567b3b6917b2cd)
docs: Add important remarks on the use of the peak FCF in SC/21.
(cherry picked from commit d13e7a6726cf1c9a43cbdcfe500e54403a8706c0)
docs: Remove trailing spaces from SC/21.
(cherry picked from commit 2056db36f09e87147153db95df28723489d6355a)