locale(encoding=) should not strictly rely on iconvlist()

[bastistician]

The check_encoding() auxiliary function verifies the specified input encoding via iconvlist()

readr/R/locale.R

Line 132 in e529cb2

if (tolower(x) %in% tolower(iconvlist())) {

and rejects any non-listed encoding.

Unfortunately, this unnecessarily breaks locale() on platforms where iconvlist() is incomplete. As ?iconvlist says

On most platforms 'iconvlist' provides an alphabetical list of the
supported encodings. On others, the information is on the man
page for 'iconv(5)' or elsewhere in the man pages

I'd suggest to always accept the portable encodings "latin1" and "UTF-8" and otherwise only give a warning rather than stop if the specified encoding is not found.

This PR also fixes long-standing package check failures on Alpine Linux, e.g.,

--- re-building ‘locales.Rmd’ using rmarkdown
Quitting from lines  at lines 141-166 [unnamed-chunk-12] (locales.Rmd)
Error: processing vignette 'locales.Rmd' failed with diagnostics:
Unknown encoding latin1
--- failed re-building ‘locales.Rmd’