This function checks that the specified item, such as a NINO, business
id, tax year, savings account name etc is in the correct format (not
necessarily that they are actually valid).

Note: The postcode check isn't a strict postcode format check, it just
checks against what HMRC says is valid for a postcode. E.g. from
<https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/property-business-api/6.0/oas/page#tag/Annual-Submissions/paths/~1individuals~1business~1property~1uk~1%7Bnino%7D~1%7BbusinessId%7D~1annual~1%7BtaxYear%7D/put>

From the "Request Body schema"

  ukProperty -> allowances -> structuredBuildingAllowance -> building ->
  postcode

    string ^[0-9a-zA-Z{À-˿’}\- _&`():.'^]{1,90}$

    The postcode for the building

Use like

  bool ok;

  ok = mtd_is_valid_fmt(MTD_VLDT_FMT_NINO, "AB283902D");
  ...

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

Link: <https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/other-deductions-api/2.0/oas/page>
Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

We were checking the result of the fscanf(3), but it was returning 1
(which is probably correct for matching the requested UUID) but we were
checking it for 36, then length of the UUID.

Anyway replace fscanf(3) with fread(3), this seems better as we just
want to read the 36 byte UUID from the procfs file and don't need to do
any parsing.

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

This was somewhat broken, how I didn't notice sooner, I've no idea!

The issue was whenever we did the fstatat(2) check and then proceeded to
skip the mkdirat(2) we would also skip appending a '/' to the path so
for a path like /foo/bar we'd end up with /foobar

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

While it's harmless, e.g. //foo/bar, let's not...

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

Link: <https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/individuals-charges-api/3.0/oas/page>
Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

Check the result of the fstatat(2) to see if we got a non-directory path
component.

This would really only be a problem if the *last* component wasn't a
directory as otherwise the mkdirat(2) will catch it.

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>

Signed-off-by: Andrew Clayton <ac@sigsegv.uk>