i18n: Use %s placeholders in wp-http

[ramiy]

Old translation string:

• Could not open handle for fopen() to %s

New translation string:

• Could not open handle for %1$s to %2$s.

The function can be mistakenly changes by the translator. The best practice is to replace the fopen() function with a %s placeholder.

The patch also adds a period at the end of the sentence and adds translators comments.

[ramiy]

add <code> tags around the function name

[SergeyBiryukov]

In 41901:

I18N: Replace function name in error message in WP_Http_Curl::request() and WP_Http_Streams::request() with a placeholder.

Props ramiy.
Fixes #41666.