PVA TLS: remove keychain password-file mechanism (EPICS_PVA[S]_TLS_KEYCHAIN_PWD_FILE), keep only the ";password" postfix

[george-mcintyre]

Summary

The PVA TLS keychain code supports two ways to supply the keychain password:

1. Postfix form: EPICS_PVA[S]_TLS_KEYCHAIN=/path/to/keychain;password
2. A separate password file pointed to by EPICS_PVA[S]_TLS_KEYCHAIN_PWD_FILE

In the reference C++ implementation (PVXS), the password-file mechanism (2) has been judged an anti-feature by the maintainer and removed, keeping only the ;password postfix (1). Phoebus's PVA client implements the same two mechanisms and should make the matching change so the Java and C++ stacks stay consistent.

Upstream rationale

epics-base/pvxs PR #171 review comment:
epics-base/pvxs#171 (comment)

I thought we had agreed that this was an anti-feature? Remove.

Corresponding changes in the C++ / docs repos

• pvxs (slac-epics/pvxs-tls): removed EPICS_PVA[S]_TLS_KEYCHAIN_PWD_FILE env handling and the file-read code in src/config.cpp; kept the file;password postfix and the tls_keychain_pwd member. Commit 462e6716.
• pvxs-cms (slac-epics/pvxs-cms): removing the equivalent file-password paths for the PVACMS / CERT_AUTH / ADMIN keychains and the LDAP account, adding file;password postfix parsing where those keychains lacked it (in progress on branch fix/pr171-review-comments).
• pvxs-docs (slac-epics/pvxs-docs): documentation updated to drop the password-file ability and document the postfix form (in progress on branch fix/pr171-review-comments).

What needs to change in Phoebus

core/pva/src/main/java/org/epics/pva/common/SecureSockets.java:

• readKeychainPassword(boolean is_server) (around lines 230-250) reads the password from the file named by EPICS_PVAS_TLS_KEYCHAIN_PWD_FILE / EPICS_PVA_TLS_KEYCHAIN_PWD_FILE. This method and its call sites should be removed.
• The ;password postfix is already supported in createContext(...) (the keychain_setting.indexOf(';') split, around lines 122-135), so no new password path needs to be added; only the file-based path is removed.
• Update the class Javadoc that mentions the *_PWD_FILE variables (around lines 115-118) accordingly.

Acceptance

• EPICS_PVA[S]_TLS_KEYCHAIN_PWD_FILE is no longer read anywhere.
• EPICS_PVA[S]_TLS_KEYCHAIN=/path;password still loads an encrypted keychain.
• Existing TLS/secure-socket tests still pass.